Mass-spectrometers



J. BISHOP MASS-SPECTROMETERS Nov. 22, 1960 3 SheetsSheet 1 Filed March10 1959 4144/1 70? Ja/s A 307/01 Nov. 22, 1960 J. BISHOP 2,961,538

MASS-SPECTROMETERS Filed March 10, 1959 5 Sheets-Sheet 2 INVEN'IOR JOHNBISHOP Nov. 22, 1960 J. BISHOP 2,961,538 MASS-SPECTROMETERS Filed March10, 1959 3 Sheets-Sheet 3 INVENTOR JOHN BISHOP United States PatentOMASS-SPECTROMETERS John Bishop, Bebington, Wirral, England, assignor tothe United Kingdom Atomic Energy Authority, London, England Filed Mar.10, 1959, Ser. No. 798,387

Claims priority, application Great Britain Mar. 17, 1958 6 Claims. (Cl.25041.9)

This invention relates to collector heads for mass spectrometers.

In the analysis of the isotopic concentrations of materials using a massspectrometer it is customary to gen differing mass number. In the caseof a beam containing ions of two isotopes the beam is split into firstand second ion beams which are passed into a collector head in which thefirst ion beam is collected by a first collector electrode. and thesecond ion beam passes through a slot in the first collector electrodeand is collected by a second, collector electrode behind the firstcollector electrode.

' The slot in the first collector electrode is usually of rectangularcross-section corresponding in shape and dimensions with the second ionbeam. In order to obtain good resolution this slot is carefully alignedwith the second ion beam and generally this is done during manufactureby accurately dowelling the ion source and the collector head withrespect to the analyser tube. However in passing through the massspectrometer the ion beams are subjected to aberrations, becomedistorted and twisted so that the resolution is impaired. It is anobject of the invention to provide means compensating in part theeffects of such aberrations.

According to the invention a collector head for a mass spectrometercomprises a first collector electrode for a first ion beam, the firstcollector electrode having a slot allowing passage of a second ion beamon to a second collector electrode, and is characterised in that thefirst collector electrode is rotatable in a plane transverse to the ionbeams by an adjuster external to the collector head.

One embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

Fig. l is an end elevation.

Fig. 2 is a sectional plan along the line IIII in Fig. 1.

'Fig. 3 is a sectional elevation along the line III-III in (Fig. 1.

Fig. 4 is a detail of the section along the line IV-IV in Fig. 2.

In the drawings there is shown a mass spectrometer collector headcomprising a cylindrical chamber 1 fitted with an ion beam inlet tube 2and containing an electrode structure 3. The collector head connectswith the analyser tube 4 of a mass spectrometer through the beam inlettube 2, the analyser tube 4 and the beam inlet tube 2 being fitted withflanges 5 and 6 which are clamped and located in gas tight connection bybolts 7 and dowels 8. The chamber 1 has flanged ends 9 and 10. Thechamber 1 is connected to an evacuating system (not shown) at theflanged end 10 and is closed at the flanged end-9 by p a circular coverplate 11 clamped by nuts and bolts 12, additional location beingprovided by a dowel 13. The beam inlet tube 2 which passes through thewall of the chamber 1 is located diametrically opposite to a tube 14which also passes through the wall of the chamber 1.

The tubes 2 and 14 terminateinside the chamber 1 and the electrodestructure 3 is located between the inner adjacent ends 15 and 16 of thetubes 2 and 14." The electrode structure 3 comprises a rotatably mountedtubuj lar sleeve 17 having an external end flange 18 and mounted inabearing bush 19 which is welded to a tubular 1 pillar 20. The pillar 20is securedto the cover plate 11 I by bolts 21 and the sleeve 17 isretained in the bearing bush 18 by a locating ring 22 fitted withclamping screws 23. A circular plate 24 is fitted to the flanged endofthe sleeve 17 by bolts 26. The plate 24 supports an array of fiveparallel electrodes 27 by means of composite ceramic pillars 28 so thatthe electrodes 27 are insulated from each other and from the plate 24.The pillars 28,1

of which there'are four, comprise short ceramic spacer tubes 29 threadedon rods 30 projecting from the plate 24. The rods 30 pass-through theelectrodes 27 which;

are clamped between the spacer tubes 29 by nuts 31 screwed on the rods30. The electrodes 27 areof three types, viz: a central collectorelectrode 32 located between two secondary electron suppressor rings 33and two guard rings 34. The plate 24 and the secondary electronsuppressor rings 22 have central rectangular apertures 35 while theguard rings 34 have central circular apertures 36. The collectorelectrode 32 has a centrally placed rectangular slot 37 as shown in Fig.4. V 1

A slide 38 movable in guides 39 brazed to the collector. electrode 32has a rectangular slot 40 corresponding to thei slot 37 in the collectorelectrode 32 so that the etfective; area of the slot 37 may be varied bymoving the slide 38. The slide 38 is movable under the action of a push'rod 41 which after passing through the plate 11, extends through thetubular pillar 20 and is coupled with the slide 38 by a hook 42 on therod 41 which engages with a groove 43 in a thickened part 44 of theslide 38. v The push rod 41 is sealed in passage through the plate 11 bya bellows type seal 45 which accommodates reciprocating movement of thepush rod 41. The seal 45 comprises movable and fixed bellows flanges 46and 47 joined by a collapsible metal bellows 48. The bellows flange 47is secured to the plate 11 by bolts 49 and the movable bellows flange 46is slidable in a flanged tube 50 carried on the fixed bellows flange 47.Rotation of the bellows flange 46 in the tube 50 is prevented by a pin51 carried by the flange 46 and engaging with an internal keyway 52 inthe tube 50. The tube 50 is externally threaded to fit a knurledadjusting nut 53 having an internal flange 54 with a lip 55 adapted tothrust against and thereby move the bellows flange 46 on rotation of thenut 53 in a clockwise sense. The bellows flanges 46 and 47 are springloaded apart by a compression spring 56 which.

bears against the movable bellows flange 46 through a thrust washer 57.The push rod 41 is sealed and electrically insulated in passage throughthe movable bellows flange 46 by a glass to metal seal 58. The seal 58comprises two separate cylindrical glass sections 59 and 60 which arejoined by a short metal guard tube 61. The seal 58 is brazed at a metaltube 62 to the movable bellows flange 46 and at a metal tube 63 to thepush rod 41.

A second collector electrode 64 of tubular form is located coaxiallyinside the tube 14 adjacent to the electrode structure 3. The electrode64 is fixed to a rod 65 by means of a tubular bush 66 fitted with alocking screw 67. The tube 14 is closed by a circular cover plate 68which is clamped by nuts and bolts 69 to a flange 70 welded to theexternal end 71 of the tube 14. The

Patented Nov, 22,1960

rod 65 is sealed and insulated in passage through the cover plate 68 bya glass to metal seal 72 identical to the seal 58.

Referring to Fig. 3 rotation ofthe electrode structure 3 is eifectedfrom outside the collector head by means of a push rod 73. A pivot pin74 couples the push rod 73 with a lug 75 formed integrally with theflange 18 on the rotatable sleeve 17. The push rod 73 passesout of thecollector head through a hole 76 in the cover plate 11 and is welded inconnection with a stem 77 integral with a movabe bellows flange 78forming part of a bellows type seal 79. A fixed bellows flange 80attached to the cover plate 11 by bolts 81 is joined with the movablebellows flange 78 by a collapsible metal bellows 82. The movable bellowsflange 78 is slidable in a flanged tubular thimble 83 attached by thebolts 81 to the fixed bellows flange 80. A micrometer head 84 having abody 85 and a stem 86 is supported by the thimble 83. The thimble 83 hasan end bush 87 which accommodates one end of the body 85 of themicrometer head 84. The stem 86 of the micrometer head 84 passes throughthe bush 87 and is coupled with the movable bellows flange 78 by a ballrace 88 comprising inner and outer members 89 and 90. The movablebelTows flange 78 is drilled to accommodate the outer member 90 of theball race 88 while the inner member 89 of the ball race 88 is fittedwith a pin 91 which is screwed into the stem 86 of the micrometer head.The outer member 90 of the ball race 88 is retained in the movablebellows flange 78 by an annular plate 92 fixed by bolts 93.

In use of the collector head in conjunction with a mass spectrometer twoseparate isotope beams of nominal rectangular cross section andcontaining ions of diiferent mass number are passed from the masssepectrometer analyser tube 4 into the collector head. It is arrangedthat one of the beams impinges on the collector electrode 32 and theother passes through the slot 37 in the collector electrode 32 and istrapped by the tubular collector electrode '64. The beam to be collectedby the tubular electrode 64 is aligned axially with the slot 37 byadjustment of the magnetic field in the mass spectrometer. In order toaccommodate for twisting of the ion beam the slot 37 is adjustedangularly with respect to the beam by means of the micrometer head 84which operates the push rod 73 to rotate the sleeve 17 and hence theelectrode structure 3. In order to accommodate for any distortion in thewidth or shape of the beam the width of the slot 37 is adjusted bymoving the slide 38. The slide 38 is moved by rotating the knurled nut53 to move the push rod 41 with flexure of the bellows seal 45. The hook42 and the groove 43 engagement between the rod 41 and the slide 38accommodates rotational movement of the electrode structure 3.

The guard rings 34 are provided in the electrode structure 3 to preventleakage of D.C. currents onto the collector electrode 32 while thesecondary electron suppressor rings 33 are held at a negative potentialwith respect to the collector electrode 32 and act to suppress electronsemitted by the collector electrode 32 due to the ionic bombardmentthereof. The guard tube 61 of the metal to glass seal 58 is connected toa feed back line from a vibrating reed electrometer which provided ameasurement of the ion current of the collector electrode 32 bymeasuring the potential drop across a high resistance through which thepush rod 41 is connected to earth. The connection of the guard tube 61with the electrometcr feed back reduces leakage currents across theglass of the glass to metal seal 58. The guard tube 61 in the glass tometal seal 72 is similarly connected to the feed back line of avibrating reed electrometer measuring the ion current of the collectorelectrode 64.

I claim:

1. A collector head for a mass spectrometer comprising afirst collectorelectrode for a first ion beam, the first collector electrode having aslot allowing passage of a second ion beam on to a-second collectorelectrode, characterised in that the first collector electrode isrotatable in a plane transverse to the ion beams by an adjuster externalto the collector head.

2. A collector head as claimed in claim 1, characterised in that saidfirst collector electrode is mounted in a rotatable sleeve and saidadjuster comprises a rod for rotating the sleeve, the rod being coupledto the sleeve and sealed in passage through the collector head by abellows.

3. A collector head as claimed in claim 1, characterised in that amovable slotted mask is provided for adjusting the width of the firstcollector electrode slot, the slotted mask being movable by a secondadjuster external to the collector head.

4. A collector head as claimed in claim 2, characterised in that amovable slotted mask is provided for adjusting the width of the firstcollector electrode slot, the slotted mask being movable by a secondadjuster external to the collector head.

5. A collector head as claimed in claim 3. characterised in that saidslotted mask is movably mounted on said first collector electrode andsaid second adjuster comprises a rod coupled to the slotted mask andsealed in passage through the collector head by a bellows, the couplingbetween the slotted mask and the rod accommodating rotational movementof said first collector electrode.

6. A collector head as claimed in claim 4, characterised in that saidslotted mask is movably mounted on said first collector electrode andsaid second adjuster comprises a rod coupled to the slotted mask andsealed in passage through the collector head by a bellows, the couplingbe tween the slotted mask and the rod accommodating rotational movementof said first collector electrode.

No references cited.

